<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{
            margin: 0;
            padding: 0;
        }
        ul li {
            list-style: none;
            width: 200px;
            height: 50px;
            /* flex-shrink: 1; */
            /* flex-grow: 1; */
            flex: 1 1 200px;
        }
        
        ul {
            width: 700px;
            display: flex;
        }
    </style>
</head>
<body>
    <ul>
        <li style="background: red;">1</li>
        <li style="background: greenyellow;">2</li>
        <li style="background: blue;">3</li>
    </ul>
</body>
</html>